1. Field of the Invention
This invention relates to a novel tolan derivative and a liquid crystal composition containing the same.
2. Description of the Related Art
Display elements applying liquid crystals utilize various electrooptical effects of liquid crystal substances, and the display mode thereof include various modes such as TN (Twisted Nematic) mode, DS (Dynamic Scattering) mode, guest-host mode, DAP mode (Dynamic Scattering), etc. The properties required for liquid crystals used according to these modes vary, respectively, but it has been required in common to the modes that the liquid crystals exhibit liquid crystal phases within a temperature range as broad as possible and they are stable to moisture, heat, air, etc. At present, however, there is no single compound which satisfies all such requirements and hence there have been used liquid crystal compositions obtained by mixing several kinds of liquid crystal compounds with each other or by mixing several liquid crystal compounds with compounds similar to liquid crystals.
As examples of tolan derivatives used as a component of liquid crystal materials, compounds expressed by the following formulas are disclosed in (1) Japanese patent application laid-open No. Sho 60-155142/1985 and (2) Japanese patent application laid-open No. Sho 61-260031/1986, respectively: ##STR2## wherein R' represents an alkyl group and X represents a halogen atom, and ##STR3## wherein R" represents an alkyl group; X.sub.1 represents a hydrogen atom or a halogen atom; and X.sub.2 represents a halogen atom. These tolan compounds are liquid crystal materials characterized by having a halogen atom(s) as a substituent and also having a large optical anisotropy value (hereinafter abbreviated to .DELTA.n).
Among the compounds, the compound of the formula (1) has a relatively high clearing point, but has a low compatibility with other liquid crystal compounds at low temperatures so that crystals are deposited and hence it has a drawback of being impossible to make the best use of its specific features. The compound of the formula (2) has no liquid crystal phase; hence when it is used as a component of liquid crystal compositions, it has a drawback of making the N-I point of the compositions too low.